avast! Free Antivirus is FREE - you only need to register it with an email address once per year.
You can try paid-for versions of avast! software for 30 days, free of charge. During this trial period, you have full access to all features and technical support. You may then buy the software license or uninstall the trial program - it's up to you.
For standard installations, use the regular (Administration Console) installation file. For non-standard installations (e.g. if you need to install the administration console on multiple computers) use the (Administration Console including Mirror) installation file, which includes the most recent version of the avast! Mirror.